Modulating the Electromechanical Response of Bio-Inspired Amino Acid-Based Architectures through Supramolecular Co-Assembly

Supramolecular packing dictates the physical properties of bio-inspired molecular assemblies in the solid state. Yet, modulating the stacking modes of bio-inspired supramolecular assemblies remains a challenge and the structure–property relationship is still not fully understood, which hampers the rational design of molecular structures to fabricate materials with desired properties. Herein, we present a co-assembly strategy to modulate the supramolecular packing of N-terminally capped alanine-based assemblies (Ac-Ala) by changing the amino acid chirality and mixing with a nonchiral bipyridine derivative (BPA). The co-assembly induced distinct solid-state stacking modes determined by X-ray crystallography, resulting in significantly enhanced electromechanical properties of the assembly architectures. The highest rigidity was observed after the co-assembly of racemic Ac-Ala with a bipyridine coformer (BPA/Ac-DL-Ala), which exhibited a measured Young’s modulus of 38.8 GPa. Notably, BPA crystallizes in a centrosymmetric space group, a condition that is broken when co-crystallized with Ac-L-Ala and Ac-D-Ala to induce a piezoelectric response. Enantiopure co-assemblies of BPA/Ac-D-Ala and BPA/Ac-L-Ala showed density functional theory-predicted piezoelectric responses that are remarkably higher than the other assemblies due to the increased polarization of their supramolecular packing. This is the first report of a centrosymmetric-crystallizing coformer which increases the single-crystal piezoelectric response of an electrically active bio-inspired molecular assembly. The design rules that emerge from this investigation of chemically complex co-assemblies can facilitate the molecular design of high-performance functional materials comprised of bio-inspired building blocks.

with a data spacing of 0.5 nm at ambient temperature. CD spectra were obtained by subtracting the blank background.
Fourier-transform infrared (FTIR) spectroscopy. The single and co-crystals (Ac-L-Ala, Ac-D-Ala, Ac-DL-Ala, BPA, BPA/Ac-L-Ala, BPA/Ac-D-Ala, BPA/Ac-DL-Ala) were deposited onto a disposable crystal KBr IR card, (International Crystal Labs, Garfield, New Jersey, USA). The FTIR spectra were collected using a Nicolet iS50 FTIR spectrometer (Thermo Scientific, Waltham, Massachusetts, USA) from 4000 to 400 cm -1 . The background signal was recorded without a sample and subtracted to obtain each FTIR spectrum.
Processing and structural refinement of crystal data. The diffraction data were analyzed using the Bruker Apex2 suite. The structure was solved by direct methods using SHELXT-2014/5.59. The refinements were measured with SHELXL-2016/4 and weighted full-matrix least-squares against |F 2 | using all data. Atoms were refined independently and anisotropically, with the exception of hydrogen atoms, which were placed in calculated positions and refined in riding mode. Crystal data collection and refinement parameters are shown in Table S1 and the complete data can be found in the cif file as supplementary information.
Atomic force microscopy (AFM) nano-indentation experiments. All the AFM nanoindentation experiments were performed using a commercial AFM (JPK, Nanowizard IV, Berlin, Germany). QI mode (conditions: pixels: 126 × 126; Z length: 0.3 μm; extend and retract speed: 30 μm s -1 ; Z resolution: 80000 Hz; maximum loading force: 800 nN) and RTESPA-525 cantilevers (Bruker Company, half-open angle of the pyramidal face of θ: < 10°, tip radius: ~10 nm, spring constant: ~200 N m −1 ) were used in the experiments. Typically, the samples were fixed on a mica substrate and the cantilever was moved above the crystals with the help of a microscope. Then, the cantilever approached the surface of the crystal and retracted, and the forcedisplacement curves were recorded during the process. The indentation depths pressed by the cantilever tip were less than 10 nm for all the samples. Young's modulus of crystals could be calculated by fitting the extended curve with the Hertz model (1).
In which F corresponds to the force, corresponds to the depth of the crystal pressed by the cantilever tip, corresponds to the radius of the tip, is the Young's modulus of the crystals and ν is the Poisson ratio (ν = 0.3). The point stiffness was calculated from the force-displacement curves after subtracting the deformation of the cantilever. For each sample, more than 6 regions were randomly selected to perform the experiments. Tip to tip dependency was excluded by using more than two cantilevers in each experiments. All the data was analyzed and the two-dimensional diagrams were reconstructed using the commercial software (JPK data processing 7.0.46) provided by JPK company.
Point stiffness measurement. The cantilever and crystal in the AFM experiments could be considered as two serial springs. Thus, the point stiffness was directly calculated from the extend curves (kmeans), the stiffness of cantilever (kcan) and the crystal stiffness (kcry) following equation (2). After the calculation of crystal stiffness using equation (2), the stiffness histograms were also constructed.
Piezoelectricity prediction. Electromechanical properties were predicted from periodic density functional theory (DFT) calculations [1] on the range of BPA/AcA chiral co-crystals and their unimolecular component crystals using the VASP code [2] .
Electronic structures were calculated using the PBE functional [3] with Grimme-D3 dispersion corrections [4] and projector augmented wave (PAW) pseudopotentials [5] . The crystal structures were optimized using a plane wave cut-off of 600 eV with a 4x4x4 kpoint grid. A finite differences method was used to calculate the stiffness tensor, with each atom being displaced in each direction by ± 0.01 Å, and piezoelectric strain constants and dielectric tensors were calculated using Density Functional Perturbation Theory [6] (DFPT), with a plane wave cut-off of 600 eV and k-point sampling of 2x2x2.
Young's moduli were derived from the stiffness tensor and its inverse compliance matrix components. Values are presented as a Voigt-Reuss-Hill average [7,8] . Crystal structures were visualized using VESTA [9] . Another silicon substrate with the copper electrode was closely adsorbed on the doublesided tape to complete the device structure. No gap was left in the device. Copper wires were connected to the electrodes of the device and PDMS was coasted on the electrodes as buffer. For power generation test, the power generator filled with crystals was firmly fixed onto a stainless-steel plate. Cyclic forces were periodically applied on the device with force control. Al foil was used to avoid triboelectric signals. The electrical signal from the devices was collected using an electrometer (Keithley 6514) and a data acquisition system (NI USB-6218).       Goodness-of-fit on for Ac-L-Ala, Ac-D-Ala, Ac-DL-Ala, respectively. [11][12] Table S7. The magnitude of the predicted piezoelectric strain constants of BPA, Ac-L-Ala, Ac-D-Ala, Ac-DL-Ala, BPA/Ac-L-Ala, BPA/Ac-D-Ala, and BPA/Ac-DL-Ala. Biological materials L-alanine 6 [13] Collagen 12 [14] Hydroxyapatite 14 [15] Non-biological materials